Telecommunication networks of today generally employ optical fibers for signal transmission. Optical signals are transported long distances on optical carriers and features like long legs and power splitting necessitate amplification of weakened signals. Optical amplifiers typically comprise a comparatively short amplifier fiber doped with a rare-earth ion or another substance that is capable of fluorescing. Light from a pump source causes electrons of the rare-earth ions to jump to a temporary excited stage, and light of the input signal stimulates spontaneous emission from the excited level. The light of this emission presents the same characteristics (wavelength, polarization and direction of propagation) as the input signal and the emission results in that the gain of the input signal is increased.
The demand for increasing bandwidth, primarily caused by the tremendous growth of the Internet, is driving the rapid deployment of optical amplifiers. For the conventional (C) band, it is well known to use erbium doped fiber amplifiers (EDFA), which has been thoroughly researched. However, the increasing demand for bandwidth in wavelength division multiplexing (WDM) optical communication systems has led towards extending the transmission bands outside the C-band. Below the C-band, there is the so-called S-band (1460-1520 nm) for which the more recent thulium doped fiber amplifiers (TDFA) are suitable. There are also new EDFAs, operating at a record 25 dBm output power, which have a gain flatness of less than 0.8 dB over the L-band (1570-1610 nm).
Both EDFA and TDFA use fibers doped with rare earth ions that typically show a bandwidth of approximately 90 nm. In [1], for example, an Er3+/Tm3+ codoped silica fiber with a bandwidth from 1460 to 1550 nm is described. The maximum values of optical amplifier bandwidth presented, all kinds of glass compositions considered, lie in the range of 110-130 nm [2, 3]. In order to achieve true broadband amplification, this bandwidth is not sufficient. It would be very desirable to find a way of improving the bandwidth of optical amplifiers based on rare earth doped fibers.
Moreover, a problem associated with previous fiber types, such as fibers based on fluoride, tellurite and chalcogenide glasses, are the inferior mechanical properties thereof. Such fibers are often incompatible with the conventional silica fibers used in telecom.
Accordingly, the optical amplifier fibers of conventional telecommunication systems are far from satisfactory and there is a considerable need for an improved glass composition allowing broadband amplification of optical signals.